Spectrometer for measuring inelastically scattered light

ABSTRACT

The spectrum of light, inelastically scattered by a sample ( 16 ) is measured. The light is guided through a capillary ( 12 ) from and to the sample, at least in one of these directions, through the channel no inelastic scattering of light occurs which can form an interfering background when measuring on the sample. By guiding the light through the channel, inelastic scattering of this light is prevented and it becomes possible to guide scattered light back through the channel to spectral analysis equipment ( 14 ) without problems with inelastic scattering during the guidance of the light. Preferably, the light is guided through the channel of the capillary in both directions.

[0001] The invention relates to a spectrometer for measuringinelastically scattered light. Raman spectrometers are an example ofspectrometers for measuring inelastically scattered light. When a sampleis irradiated with monochromatic light, the light scattered by thesample will contain wavelength components different from those presentin the incident light. So-called Raman scattering of light on moleculespresent in the sample creates this effect. With a Raman spectrometer, asample is illuminated with a strong, essentially monochromatic lightsource. The light which is scattered by the sample is collected and thespectral distribution is analysed for spectral components with awavelength different from that of the light source.

[0002] U.S. Pat. No. 5,862,272 describes the use of optic fibres forguiding Raman spectra of samples to and from the sample, while thesample is at a distance from the light source which is used forexcitation and/or at a distance from the equipment for detection of theRaman signal. The use of optic fibres enables measurements at locationswhich are otherwise difficult to reach or cannot be reached at all, asis the case, for instance, in in vivo measurements on tissue, forinstance in blood vessel examination or more generally in endoscopicapplications, or as is the case in industrial applications such asin-line process control.

[0003] U.S. Pat. No. 5,862,273 describes the use of a filter at theextremity of the fibre (excitation-fibre) which guides the monochromaticlight from the light source to the sample, adjacent the sample to beexamined. This filter (excitation filter) only transmits light of thewavelength of the light source and no Rams scattered light from thefibre, which is generated therein by inelastic scattering of light fromthe monochromatic light source. This enables sensitive measurement,without interference by inelastically scattered light from the fibrewhich returns to the analysis unit after scattering or reflection on thesample.

[0004] Further, the use of an excitation filter in or an the excitationfibre necessitates the use of a different fibre or fibres (collectionfibres) for collecting light scattered by the sample and its guidance tothe analysis unit, since otherwise the Raman scattered light of interestwould be stopped by the excitation filter so that it cannot reach theanalysis unit. Special measures are necessary to obtain a good overlapbetween the volume or surface of the sample irradiated by light from theexcitation fibre and the volume or surface of the sample from whichscattered light is intercepted by the collection-fibre(s). Also, the useof separate fibres for supply of light to and discharge of light fromthe sample renders Raman probes complex. The signal collectionefficiency is suboptimal and far-reaching miniaturisation is impeded.

[0005] The light scattered by the sample largely consists of elasticallyscattered light, having the same wavelength as the incident light fromthe monochromatic light source. This elastically scattered light ofconsiderable intensity can tin give rise to the generation of Ramansignal in the collection fibres, which leads to an interfering signalbackground. For that reason, for sensitive measurements, a filter(emission filter) is utilized before, on or in the collection fibre(s)on the side where the sample is located, which serves to stop the lightscattered elastically by the sample and to transmit only the scatteredlight which is shifted in wavelength with respect to the wavelength ofthe monochromatic light source.

[0006] Using excitation and emission filters limits the use of thespectrometer to a light source of a particular wavelength and/ormeasurement of the Raman signal in a particular wavelength range. Thenecessity of the use of optic filters before, in or on the fibres closeto the sample renders fibre optic probes for Raman spectroscopy complex.

[0007] Also from U.S. Pat. No. 6,208,887, a Raman spectroscope is known.This patent discusses the possibility of using a multimode laser. Thesample is illuminated via an optical fibre, without the use of opticfilters, so that the spectrometer is only suitable for lowsensitivities. For receiving scattered light the possibility of anylight guide (waveguide) is mentioned, without, however, mentioning theadvantages thereof over optic fibres.

[0008] It is one object of the invention to provide a spectrometer forinelastic scattering with a simplified construction of the probe.

[0009] It is another object of the invention to provide such aspectrometer wherein further-reaching miniaturisation of the probe ispossible.

[0010] It is a further object of the invention, among still furtherobjects, to provide such a spectrometer which is designed such thatsensitive measurements are possible, while no filters are required orfewer filters than in the prior art.

[0011] The spectrometer according to the invention is described in claim1. This spectrometer utilizes a probe with a capillary with a channelthrough which at least one of the light transports of the light from thelight source and the light scattered by the sample is guided. The otherlight transport can take place through the same channel or through awall of the capillary if it is made of transparent material. Within thecontext of this invention, a capillary is understood to mean a flexibleor non-flexible tube or pipe of any material (materials), with one ormore channels in the longitudinal direction of the tube or pipe, whichmay be evacuated or filled with a gas (gasses) or a liquid (liquids).

[0012] Due to this construction, the probe is very compact. Onecapillary can suffice for guiding light to and from the sample. Thelocation where the light leaves the capillary to illuminate the sampleand the location where the light returns scattered from the sample intothe capillary are in each other's immediate vicinity. No complexconstructions are required for creating an overlap between the area ofthe sample which is irradiated and the area from which light returns tothe analysis unit. Moreover, for the light transport through the channelin sensitive measurements, no filter is required because in the channel,due to the absence of glass, no interfering inelastic scatterings occur.

[0013] Preferably, at least the light from the light source is guided tothe sample through the channel in the capillary. In the prior art,inelastic scattering of this light in the light guide is the chiefsource of disturbances. By guiding at least this light through thechannel, a narrow band pass filter becomes superfluous. Preferably,however, both light transports, from and to the sample, are guidedthrough the channel. Hence, automatically, there is a complete overlapbetween the area of the sample which is irradiated and the area fromwhich light returns to the analysis unit. Moreover, thus, for sensitivemeasurements, no filters are required at the distal end of the lightguide, so that the construction remains very simple. Moreover, whenfilters can be dispensed with, use of the probe is not limited to aparticular wavelength. Ultimate miniaturisation of Raman probes ispossible when only one light guide needs to be utilized for supply anddischarge of light to and from the sample.

[0014] For manufacturing the hollow light guide according to thisprinciple, different techniques can be used. A smooth metal coating canbe applied on the inside of a capillary, optionally with a dielectriccoating thereon to enhance the resection. Also, an accumulation ofdielectric layers can be used, which, together, form a dielectric mirrorwith a high reflection efficiency. Further, a hollow metal capillary canbe utilized, whose inside wall is smooth to such an extent that specularreflection is ensured.

[0015] In one embodiment, a capillary is filled with a liquid of ahigher refractive index than the material of which the capillary ismade, so that for light in the liquid propagating in a certain solidangle through the capillary, total internal reflection occurs at theliquid-capillary transition and the arrangement functions as an opticalwaveguide. The liquid in the present invention is preferably confined inthe capillary, at least at the distal end (adjacent the sample), so thatit cannot be easily replaced without opening the capillary. Preferably,the liquid is selected such that it exhibits no significant Ramanscattering in the spectral region relevant to the analysis of thesample. This in contrast with optical glass fibres which in large partsof the spectrum cause a continuous high, often broadband signalbackground. Further, a coating of low refractive index can be applied tothe inside of the capillary, thus enabling total internal reflection.

[0016] It should be noted here that measuring Raman scattering onliquids in a capillary is known per se. This involves scattering on theliquid in the capillary and not on a sample located beyond the extremityof the capillary. In this known measurement on the liquid, scattering oflight from the capillary on a sample outside the capillary is undesired.

[0017] The use of hollow light guides based on this invention as a basisfor Raman probes renders the construction of these probes simple andincreases the possibilities for simple adaptation of probes toapplication-specific requirements, with regard to the light source to beused and properties of the sample to be examined as well as with regardto spatial resolution and location of the measured volume, by usingspecific optical elements at the distal extremity of the Raman probe.

[0018] An example of such provisions is a cone at the extremity of thelight guide and tapering towards the sample, which makes it possible tocapture light coming from the sample at a very large solid angle,whereupon in the cone, the solid angle is reduced, so that the light canbe more readily guided further in the capillary.

[0019] These and other objects and advantages of the spectrometeraccording to the invention will be further described with reference tothe following figures.

[0020]FIG. 1A shows a spectrometer suitable for using Raman probes basedon a hollow capillary as light guide;

[0021]FIG. 1B shows a coupling unit for light source, Raman probe andanalysis unit;

[0022]FIG. 2 shows the principle of the use for Raman spectroscopy of aprobe based on a capillary;

[0023]FIGS. 3A-3C show different embodiments of a light guide suitablefor Raman spectroscopy;

[0024]FIGS. 4-8 show different embodiments of Raman probes based onlight conducting capillaries;

[0025]FIGS. 9-13 show different embodiments of Raman probes based onthick-walled capillaries; and

[0026]FIG. 14 shows an alternative embodiment of a coupling unit forlight source, Raman probe and analysis unit.

[0027]FIG. 1A shows a sample 16 and a spectrometer for measuringscattering on the sample 16. The spectrometer includes a light source10, a probe with a hollow capillary 12, an analysis unit 14 and acoupling unit 18 which provides the coupling between the light source 10and the light guide 12 on the one hand and between wave guide 12 andanalysis unit 14 on the other hand.

[0028] In operation, with the light source 10, essentially monochromaticlight is generated, which is guided via the coupling unit into thehollow light guide 12. This light is guided through the hollow lightguide 12 to the sample 16. Light that has been scattered by the sampleis guided through the probe, preferably through the hollow light guide,to the analysis unit. In the analysis unit 14, the intensitydistribution of the light scattered by the sample 16 is measured as afunction of the wavelength. Preferably, measurements are carried out onscattered light whose frequency is shifted by scattering with a shiftexpressed in inverted centimeters between −4000 and +4000.

[0029] Optionally, the polarisation of the scattered light is measured.Optionally, only the intensity and/or polarisation of the scatteredlight in one or several selected subregions of the spectrum is/aremeasured. Optionally, the coupling unit 18 can be connected via opticallight guides (not shown) to light source 10 and analysis unit 14,respectively.

[0030]FIG. 1B shows a possible embodiment of the coupling unit 18. Lightfrom the light source 10 (represented in full line), preferably a narrowband laser which emits light of a wavelength lo, is coupled-in in anoptic fibre 11, preferably a fibre with a small numerical aperture, forinstance a monomode fibre. Light issuing at the distal end of this fibreis collimated with the aid of a lens 13 and filtered by a filter 15 sothat only the narrow band laser light l₀ is coupled-in with the aid oflens 17 into the hollow light guide 12 and reaches the sample. Thehollow light guide 12 is attached to coupling unit 14 by means of amechanic coupling 20, for instance by means of an SMA-coupling or anFC-coupling, so that simply and at will, different light guides can beattached to coupling unit 14, or be exchanged. Light which is scatteredby the sample (represented in broken lines) is guided back through thehollow light guide 12 to the coupling unit 18. After collimation by lens17, this scattered light issuing from the hollow light guide 12 reachesfilter 15. Light l_(o) not shifted in wavelength is largely transmittedthrough the filter, preferably by more than 99%. Raman scattered lightwith wavelengths l_(0,2 . . . N) is largely reflected by filter 15,preferably by more than 99%. Filter 22 provides for further suppressionof the component lo, whereupon the Raman scattered light is focussed bylens 19 onto a bundle of optic fibres 21. This bundle is for instancecircular on the side where the Raman scattered light is focussed, butdifferent shapes are possible. Preferably, the bundle is designed suchthat as much light as possible is coupled into the core of the fibres ofwhich the bundle consists. The fibres can be circular, square orotherwise multiangular. At the distal side, on which side the light iscoupled into the analysis unit 14, the fibres of fibre bundle 21 arearranged in a straight row, perpendicular to the dispersion direction ofa dispersive spectrometer.

[0031] In this embodiment, the coupling unit 14 is suitable for couplingthe hollow light guide 12 to a dispersive analysis unit, in which thewidth dimension of the row of fibres is partly determinative of thespectral resolution of the device. This is not the case when theanalysis unit 14 is a Fourier-Transform Raman spectrometer or when, withthe aid of optic fibres, the intensity of the scattered light ismeasured ill only one or several wavelength intervals. In these twocases, fibre bundle 21 can be optionally omitted.

[0032] In a different embodiment of the coupling unit, filter 15 isreplaced with a mirror which reflects the light issuing from the hollowlight guide in the direction of filter 22. The light from light source10 reaches the hollow light guide through a hole in the mirror of adiameter which is relatively small with regard to the total mirrorsurface.

[0033] In a different embodiment (not shown) laser light is coupled intothe hollow light guide via a small mirror. The surface of the mirror issmall with respect to the diameter of the light beam issuing from thehollow light guide and collimated by a lens. In still another embodiment(not shown) laser light is coupled into the hollow light guide via asmall mirror placed between a lens and the hollow light guide.

[0034]FIG. 2 shows the principle of the use of a capillary 12 as hollowlight guide for Raman spectroscopy light of wavelength l₀ from lightsource 10 (not shown) is coupled in via coupling unit 18 (not shown) incapillary 12 and is reflected on the inner wall of the capillary andreaches the sample 16. The sample scatters the incident light throughelastic and inelastic scattering. A portion of this scattered light,consisting of a strong component of, a wavelength l₀ and, in addition,components l_(1,2 . . . ,N) shifted in wavelengths, is intercepted bycapillary 12 and guided back to the coupling unit 14 (not shown) towhich capillary 12 is connected by means of a mechanical coupling 20(not shown).

[0035] FIGS. 3A-C shows different possible embodiments of the hollowlight guide.

[0036]FIG. 3A shows a capillary 32 made of a material having anintrinsically high reflection coefficient for the relevant part of theelectromagnetic spectrum, for instance of metal. The inside of thecapillary is sufficiently flat, so that specular reflection from thewalls dominates. Gold has a high reflection in the near-infrared (>800nm) of 98-99%. For applications in the ultraviolet, aluminum ispreferred. When the length of the hollow light guide remains limited,such a capillary suffices for the application described here. In alonger capillary, the reflection losses could become a drawback whensignal is intercepted from a sample which inherently emits a weaksignal. For a light ray 31, entering through the centre of the capillary32 at an angle φ to the axis of the capillary, having a radius R and alength L, and made of a material having reflection coefficient ρ, theweakening as a result of reflection losses can be assessed with the aidof formula 1.

I_(out)/I_(in)˜≅ρ^(L tan φ/2R)   (1)

[0037] For a gold capillary of a diameter of 1 mm and a length of 50 cmand light incidence at an angle φ of maximally 6°, this reflection lossis then approximately 64%.

[0038]FIG. 3B shows a capillary made of fused silica, glass or a plastic33, whose inner wall is provided with a reflecting metal coating 34. Forthe metal, in these embodiments, aluminum (particularly suitable for usein the ultraviolet), silver (for use with visible light and in thenear-infrared) or gold (near-infrared) can be utilized. The inside hasbeen smoothed such that specular reflection occurs. In a furtherembodiment a dielectric layer can be applied on the reflecting layer toincrease the reflection coefficient. Light guides according to thisprinciple are known per se which exhibit a transmission ofapproximately. 90%/m. Such light guides are suitable in particular forapplications where the light guide does not need to be bent or hardlyso. Sharp bends lead to strong decrease of the transmission.

[0039] Further, on the inner wall of the capillary, an accumulation ofdielectric layers can be applied, selected such that in the spectralregion of interest an optimum reflection is accomplished at every angleof incidence. Such a hollow light guide, whose walls function asso-called omni-directional reflector is, described in Fink, Y. et al J.Lightwave Technology 17:2039-2041 (1999).

[0040]FIG. 3C shows a capillary 35 non-absorbing in the relevantspectral region and transparent, filled will liquid 36 non-absorbing inthe relevant spectral region, having no Raman signal in the spectralregions of interest to the user. The light guide with liquid is based onthe principle of total internal reflection with which very hightransmission-efficiency can be attained, which is moreover littleinfluenced by bends in the light guide. This means that the refractiveindex of the liquid needs to be higher than that of the capillarymaterial. The numerical aperture of this light guide is determined byformula 2:

N.A.={square root}(n ₁ ² −n ₂ ²)   (2)

[0041] A possible option for liquid 36 is tetrachloroethylene. Use ofthis agent having a refractive index of ˜15.05 and only Raman scatteringat ˜1571 cm−1 and below 525 cm−1 leaves a large part of the Ramanspectrum free for measurements on a sample. When a fused silica(refractive index ˜1.46) capillary is filled with this liquid, a lightguide is formed with a numerical aperture of −0.36. Another possiblechoice is CC14. It will be clear that, depending on the application(i.e. depending, inter alia, on which part or which parts of thespectrum have to be clear of background signal contributions of theRaman probe) a particular liquid can be chosen, and that there is alarge freedom in that respect.

[0042] It will be clear that many other variations on theabove-mentioned examples are possible, such as, for instance, acapillary filled with a liquid of a lower refractive index than thematerial of which the capillary is made. The principle of light guidancethrough total internal reflection can then be realized by employing acoating of the inside wall of the capillary with a material having arefractive index lower than the refractive index of the liquid.

[0043] In principle, it is also possible to guide light through thecapillary by using total internal reflection on the transition ofcapillary material and air at the outer side of the capillary. However,this is a less advantageous solution. On the one hand, also a portion ofthe Raman signal generated in the capillary material will be guided intothe light guide. On the other hand, total internal reflection will (can)be lost at locations where the outside of the capillary is in contactwith other materials.

[0044] To obviate this last-mentioned problem, a coating of a materialwith a lower refractive index than the refractive index of the liquidcan be applied on the outside of the capillary. The disadvantage remainsthat a portion of the Raman signal generated in the capillary materialis transmitted into the liquid core. Onto the coating on the outside ofthe capillary a strongly absorbing coating has to be applied because,otherwise, total internal reflection on the transition of coating andair occurs.

[0045] In the embodiments of FIGS. 2 and 3, the Raman capillary issimply * cut off at the exit. In the case of a light guide with a liquidcore as described, a transparent sealing of the capillary will be usedat both ends. In this case, the choice of diameter of the capillarydetermines the magnitude of the measured surface and/or of the measuredvolume in the sample of which scattered light is captured.

[0046] Other embodiments of the extremity are possible. The shape of theextremity of the Raman capillary, i.e. the side of the hollow lightguide where the sample is located, can be varied. This makes itpossible, for instance, to use the Raman capillary for illumination of asample and/or signal collection from a sample at an angle or anglesdeviating from the direction of the longitudinal axis of the capillaryand/or are not centered about the longitudinal axis of the Ramancapillary. It can also be provided that control is obtained over thelocation of the measured volume from which preferentially light isintercepted and/or the spatial resolution of the Raman capillary. It canalso be provided that light scattered by the sample can be interceptedwithin a large solid angle, whereupon this solid angle is transformed toa smaller solid angle which can be (efficiently) guided through thecapillary. A number of examples are discussed in FIGS. 4 to 8.

[0047]FIG. 4 shows an embodiment of the light guide with an element 41at the distal end (the part of the probe which is in contact with thesample) which brings about a solid angle adaptation. The element 41 isfor instance manufactured from a material which gives off no or only arelatively weak Raman signal in the relevant part of the Raman spectrum.An example of such a material is UV-grade CaF₂, as can be supplied by,for instance, the firm Crystran Ltd, 27 Factory Road, Poole, Dorset BH165SL, United Kingdom.

[0048] The element 41 is for instance cone-shaped. The sides of theelement make an angle φ with the longitudinal a of the Raman capillary42. When light is intercepted by element 41 at an angle γ, larger thanφ, with the longitudinal axis of the capillary, then, after reflectionfrom the wall of element 41, the light will travel further at an angleγ′ with the longitudinal axis of the capillary which is smaller than theoriginal angle of incidence γ. When angle γ is large, more reflectionswith the sides of element 41 can occur before the light proceeds in theRaman capillary 42. Many variations on the shape of element 41 with aview to adjusting the solid angle at which intercepted signal is guidedfurther, are possible. For Raman capillaries in which use is made of areflecting coating on the inside wall of capillary, this has the greatadvantage that this limits the number of reflections of the interceptedRaman light against the walls of the hollow light guide en route to thedetector, and hence also reflection losses. For Raman capillaries inwhich use is made of total internal reflection, this has the advantagethat signal from the sample can be intercepted at a larger solid anglethan could be guided further by the Raman capillary.

[0049] Also at the detector side of the hollow light guide such a coneor an optimized non-imaging parabolic light collector can be attached.There, it serves as a numerical aperture adapter, for the analysis unit.

[0050]FIG. 5 shows a light guide with a tapering hollow element 51 witha reflecting wall at the distal end which effects a similar sort ofsolid angle adjustment to element 41 in FIG. 4.

[0051]FIG. 6 shows a light guide with a reflecting element 61 at thedistal end having for an object to make the light guide view sideways.This reflecting element can for instance be a microprism with areflecting layer 62 applied thereon. Various other embodiments arepossible.

[0052]FIG. 7 shows a capillary 70 whose extremity is depicted through alens 71 in a volume of the sample 72 or on a surface of the sample 72.In particular the light which is scattered in the same volume of thesample or on the same part of the surface of the sample is, again,coupled-in with great efficiency in capillary 70. In this way, in asimple manner, a confocal measurement arrangement is created with whichaccurate control is obtained over which part of the sample is examined.The choice of the lens and capillary deter together determine thespatial resolution of a probe in this embodiment. Also, this renders itpossible to place an extra window 73 at the extremity of the Ramanprobe, which falls outside the measured volume of the probe, andtherefore does not cause an interfering Raman signal background. For thesame reason, with such a probe, it becomes possible to measure a samplethrough a transparent container wall 74 without measuring an interferingRaman signal from the wall material. The housing 75 of this embodimentcan be adapted to the specific application.

[0053]FIG. 8 shows an embodiment as in FIG. 7 in which the lens isreplaced by a gradient index lens 80. Lens and core diameter ofcapillary 81 together are determinative of the spatial resolution.

[0054]FIG. 9 shows the use of a thick-walled capillary 90 as a guide forlight to and from a sample to be examined. On the outside, the capillaryis coated with a material 91 of lower refractive index than the materialof the capillary, while the layer thickness of this coating issufficiently great for the effects on the interface between capillarymaterial and coating to be determined by the bulk properties of thecoating material. The capillary is filled with a liquid 92 of a higherrefractive index than the material of which the capillary is made. Lightfrom light source 10 (not shown) is introduced into the capillary withina solid angle which is smaller than the maximum solid angle at which theliquid core can guide light (given by formula 2). Further, the liquid 92is selected such that in the spectral region relevant to themeasurement, no significantly interfering overlap occurs between thescattering spectrum of the sample and the scattering spectrum of theliquid. Through this suitable choice of liquid and wall material, alight guide is created through which light from light source 10 (notshown) can be guided to the sample to be examined. Light scattered bythe sample can be guided back through the wall of the capillary to theanalysis unit 14 (not shown), via a Coupling unit 18 (not shown). At thetop and bottom, the capillary is closed off with the aid of thintransparent windows 93, 94. If the Raman spectrum of the capillarymaterial overlaps the spectrum of the sample to be examined in asignificantly interfering manner, a filter 95 can be provided on thecapillary on the side of the sample, which filter screens lightelastically scattered by the sample. Accordingly, this prevents aninterfering Raman signal being generated in the wall of the capillary.Eligible for serving as a substrate for the filter is a thin, smoothlypolished disk sawn off from the capillary having the same dimensions.Such a filter can then be positioned and fastened onto the capillary ina simple manner; The refractive indices of liquid and capillary materialdetermine the solid angle at which light from light source 10 (notshown) can be coupled-in into the light guide and at which the samplecan be illuminated, according to formula 2 for the numerical aperture ofa light guide.

[0055] In the same manner, the solid angle at which light scattered bythe sample can be intercepted and guided further to the coupling unit 18(not shown) is determined by the refractive indices of the capillarymaterial and the coating on the outside of the capillary.

[0056] In an alternative embodiment, the capillary is coated on theoutside with a highly reflective material, preferably a gold or silvercoating or an omni-directionally reflecting dielectric coating.Optionally, depending on the application, a coating can be dispensedwith, so that total internal reflection can occur at the transition ofcapillary and air.

[0057] In a different embodiment, conversely, the excitation light isguided through the wall of the capillary to the sample, with optionalfiltering at the extremity of the capillary, and light scattered by thesample is guided via a channel of the capillary to the analysis unit.

[0058]FIG. 10 shows a further embodiment, in which the capillary is notfilled with a liquid but in which the inside wall of the capillary hasbeen made strongly reflective, for instance through application of asilver layer and a dielectric layer 100.

[0059]FIG. 11 shows a further embodiment in which the inside of thecapillary is coated with a material 110 of lower refractive index thanboth the liquid in the core of the capillary and the material of thecapillary. In this manner, the liquid core of the Raman capillarythrough which light from the light source 10 (not shown) is guided tothe sample and the wall of the capillary through which light Ramanscattered by the sample is discharged to the analysis unit 18 (notshown) are optically separated from each other.

[0060]FIG. 12 shows a further embodiment, in which the capillaryaccording to FIG. 9, 10 or 11 is provided with an element 121 on theside where the sample to be examined is located. Use of this elementprovides for optimal overlap between irradiated sample-volume andsample-volume from which light is intercepted. Further, the elementserves for conversion of the large solid angle at which light scatteredby the sample is intercepted, to a smaller solid angle, which light canbe guided back through the capillary in the direction of the analysisunit 18 (not shown). The element has, for instance, the shape of a cone,but other shapes are also possible. Preferably, the element ismanufactured from a material which does not emit a significantlyinterfering Raman signal in the relevant spectral range. For severalapplications, for instance UV-grade calcium fluoride is a good option,because this material has a strong Raman band only at 312 cm−1. Theelement can be coated with a strongly reflecting coating, for instancegold, or with a material of lower refractive index so that totalinternal reflection occurs. A suitable choice is, for instance, TeflonAF (produced by DuPont) having a refractive index of 1.29-1.31.

[0061]FIG. 13 shows a further embodiment in which the capillaryaccording to FIG. 10 or 11 is provided with an element 131 as in FIG.12, but for which a 10 same capillary as of which the light guide ismade serves as base material, which is subsequently brought into thedesired shape by grinding and polishing or in a different mater. Thisoffers advantages in view of a simpler manner of production, which canbe utilized if the Raman signal background of the capillary materialwhich is generated in the element does not have a significantlyinterfering effect. Additionally, this embodiment offers the possibilityof displacement of the measured volume to a location deeper into thesample, since signal does not necessarily need to be intercepteddirectly from the surface of the sample (for instance when the inside ofthe element is coated with a reflecting metal layer).

[0062]FIG. 14 shows an alternative embodiment of coupling unit 18. Theextremity of the light guide 12 is coupled to an element 140 whichreduces the solid angle at which light 146 issues from the hollow lightguide, which enables this light being coupled-in and guided further withthe aid of standard optic glass fibres 141 with, for instance, anumerical aperture of 0.22. A filter 142 serves for blocking light ofthe same wavelength as the light from light source 10 (not shown).Through a relatively saw hole in the filter, light 147 is coupled infrom light source 10 (not shown) in capillary 143. This capillary canhave one of the embodiments as described hereinabove or derivedtherefrom. Elements 140 can for instance be manufactured by taking as abasis so-called fibre optic ferrules, as sold, for instance, by Vitrocom(Mountain Lakes, N.J., USA). The inside wall is made stronglyreflective, for instance by applying a gold or silver coating 144.

[0063] In an alternative embodiment, element 140 is filled with a liquid145 of a higher refractive index than the wall material of element 140,so that total internal reflection occurs at the transition betweenliquid and wall material.

1. A spectrometer for measuring inelastically scattered light, providedwith a light source; an analysis unit for analysis of a spectraldistribution of light from the light source which has been scattered bya sample; a probe provided with a capillary through which light from thelight source is guided to the sample and scattered light is guided fromthe sample to the analysis unit, while the capillary has a wallcomprising a channel, so that for light at a transition from the channelto the wall internal reflection occurs, while the capillary is connectedsuch that the channel transports light from the light source and/or thescattered light which is guided to the analysis unit.
 2. A spectrometeraccording to claim 1, wherein at least virtually all the light from thelight source which is guided to the sample is guided through thechannel.
 3. A spectrometer according to claim 2, wherein also at leastvirtually all the scattered light is guided through the channel.
 4. Aspectrometer according to claim 1, wherein at an extremity of thecapillary on a side of the sample, provisions are made for manipulatinga spatial distribution of the light which is guided from or to thesample via the exit.
 5. A spectrometer according to claim 1, wherein thechannel is filled with a liquid of higher refractive index than the wallof the capillary.
 6. A spectrometer according to claim 1, whereinbetween the wall and the channel a mirroring metal coating is applied.7. A spectrometer according to claim 6, wherein a dielectric coating isapplied to the metal coating.
 8. A spectrometer according to claim 1,wherein between the wall and the channel, an accumulation of dielectriclayers is provided which act in a reflecting manner in the spectraloperating range of the spectrometer.
 9. A spectrometer according toclaim 1, wherein a first boundary surface of the wall and/or a secondboundary surface of the wall with an environment are coated with amaterial of a refractive index smaller than that of the wall between theboundary surfaces and/or, if the channel is filled with a liquid,smaller than that of the liquid.
 10. A probe for use in a Ramanspectrometer, provided with a first connection for a light source and ananalysis unit, a second connection for illuminating a sample to beexamined with the light of the light source and intercepting scatteredlight from the sample, a light guide for guiding light between theconnections, wherein the light guide comprises a capillary with achannel and a wall, so that for light at a transition of the channel tothe wall, internal reflection occurs, wherein the capillary is connectedto the connection such that the channel transports light between theconnections in at least one direction.
 11. A probe according to claim10, wherein at least virtually all the light of the light source whichis guided to the sample is guided through the channel.
 12. A probeaccording to claim 11, wherein also at least virtually all the scatteredlight is guided through the channel.
 13. A probe according to claim 10,wherein at an extremity of the light guide on a side of the exitprovisions are made for manipulating a spatial distribution of the lightwhich is guided from or to a sample via the exit.
 14. A probe accordingto claim 10, wherein the channel is filled with a liquid of higherrefractive index than the wall of the capillary.
 15. A probe accordingto claim 10, wherein between the wall and the channel is a mirroringmetal coating is applied.
 16. A probe according to claim 10, wherein adielectric coating is applied on the metal coating.
 17. A probeaccording to claim 10, wherein between the wall and the channel anaccumulation of dielectric layer is provided which act in a reflectingmanner in the spectral operating range of the spectrometer.
 18. A probeaccording to claim 10, wherein a first boundary surface of the walland/or a second boundary surface of the wall with an environment arecoated with a material of refractive index smaller than that of the wallbetween the boundary surfaces and/or, if the channel is filled with aliquid, smaller than that of the liquid.
 19. A method for measuring aspectrum of light inelastically scattered by a sample, wherein bothlight from a light source to be scattered and scattered light are guidedthrough a capillary, respectively from a light source to a sample andfrom the sample to an analysis unit, which capillary is designed suchthat for light from the light source on a transition of a channel in thecapillary and a wall of the capillary reflection occurs during guidanceof the light from the light source to the sample, wherein the light tobe scattered and/or the scattered light is guided through the channel.20. A method according to claim 19, wherein the light to be scattered isguided through the channel.
 21. A method according to claim 19, whereinboth the light to be scattered and the scattered light are guidedthrough the channel.
 22. A method according to claim 19, wherein thechannel is filled with a liquid.